class 12 chemistry Aldehyde ,ketone and carboxylic acid notes class 12 chemistry

1. Aldehydes, Ketones and
Carboxylic Acids
Unit 12
CLASS 12

2. Aldehydes and Ketones are characterised by the presence of Carbonyl group >C = O in their molecules.
They are functional isomers of each other and collectively called as carbonyl compounds (C=O).
Introduction
Nature of Carbonyl Group
Aldehydes, ketones and carboxylic acids are the compounds containing carbon-oxygen double bond
(>C=O) called carbonyl group. The carbon and oxygen of the carbonyl group are sp2 hybridised and the
carbonyl double bond is comprised of one σ-bond and one π-bond. The π electron cloud is present
above and below the plane and is present between carbonyl carbon and oxygen.
Due to high electronegativity of oxygen compared to carbon, the carbonyl group is polar and the dipole is
formed.

3. Aldehydes
Aldehydes are the organic compounds in which carbonyl group is attached to one hydrogen atom
and one alkyl or aryl group.
Where R can be an alkyl or aryl group.
Ketones
Ketone are the organic compounds in which carbonyl group is attached to two alkyl group or aryl group or
both alkyl and aryl group.
Where R and R’ may be alkyl or aryl groups

4. Nomenclature of Aldehydes
IUPAC naming system:
• In IUPAC system, the suffix ‘e’ of alkane is replaced by the ‘al’.
For example:
HCHO CH3CHO
Methanal Ethanal
Common naming system:
The common names of aldehydes are derived from the common names of the corresponding
carboxylic acids in which the ending ‘-ic’ is replaced with ‘-aldehyde’of acid with aldehyde.
For example:
HCHO CH3CHO
Formaldehyde Acetaldehyde

8. Nomenclature of Ketones
IUPAC naming system:
In IUPAC system, the suffix ‘e’ of alkane is replaced by the ‘one’.
For example:
Common naming system:
The common names of ketones are derived by naming two alkyl or aryl groups bonded to the carbonyl group.
For example:

9. Alkyl phenyl ketones are usually named by adding the acyl group as prefix to phenone.
For example:
Preparation of Aldehyde and Ketones
(a) By oxidation of alcohols:
Oxidation of primary and secondary alcohols in presence or oxidizing agent like
K2Cr2O7/H2SO4, KMnO4. CrO3 gives aldehydes and ketones respectively.

10. (b) By dehydrogenation of alcohols:
(c) By ozonolysis of alkenes:
Ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes, ketones
or a mixture of both depending on the substitution pattern of the alkene.

11. (d) By hydration of alkynes:
This is a commercial method to prepare ethanal.
All other alkynes gives ketones.

12. (e) By Rosenmund reduction:
Hydrogenation or acyl chloride over palladium on barium sulphate gives aldehyde.
(f) From nitriles (RCN):
This reaction is called Stephen reaction.

13. Preparation of Benzaldehyde and Acetophenone
(a) By oxidation of methylbenzene:

14. Physical Properties of Aldehydes and Ketones
1. Methanal (HCHO) is a gas at room temperature. and its 40% aqueous solution
is known as formalin. It is a reducing agent in silvering of mirrors and
decolourising vat dyes.
2. Ethanal (CH3CHO) is a volatile liquid. Other aldehydes and ketones are liquid
or solid at room temperature.
3. The boiling point of aldehydes and ketones are higher than hydrocarbons and
ethers of comparable molecular mass due to high magnitude of dipole-dipole
interactions.
4. Aldehydes and ketones have lower boiling point than those of alcohols of
similar molecular masses due to absence of intermolecular hydrogen bonding.
5. The lower members of aldehydes and ketones are miscible with water due to
the formation of hydrogen bond with water. However, the solubility decreases with
increase in length of alkyl chain.
6. Acetophenone is a hypnotic (sleep producing drug) so used as a medicine
under the name hypnone.

16. [Fehling solution is a mixture of Fehling solution A and Fehling solution B in 1: 1
ratio. Fehling solution A is aqueous copper sulphate and Fehling solution B is
alkaline sodium potassium tartrate which is also called, Rochelle salt.]

17. (c) Benedict solution With it, aldehydes (except benzaldehyde)
also give red ppt. of CU2O.
(d) Schiff’s reagent It is an aqueous solution of magenta or pink
coloured rosaniline hydrochloride which has been decolourised by
passing SO2, Aldehydes give pink colour with this reagent but
ketones do not.
Haloform reaction Aldehydes and ketones having at east one
methyl group [3-α hydrogen] linked to the carbonyl carbon atom
(methyl ketones) are oxidised by sodium hypohalite to sodium salts
of corresponding carboxylic acids having one carbon atom less than
that of carbonyl compound. The methyl group is converted to
haloform.
Iodoform reaction with sodium hypoiodite is also used for the detection of CH3 –
group or CH3CH(OH)- group by producing yellow solid CHI3.

18. Physical Properties of Carboxylic Acids
1. Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids at
room temperature with unpleasant odours. The higher acids are wax like solids.
2. The lower carboxylic acids are freely miscible with water due to the presence
of intermolecular hydrogen bonding with H2O molecules. However, the solubility
in water decreases gradually due to increase in the size of alkyl group.
3. Monocarboxylic acids have higher boiling points as compared to the alcohols
of comparable molecular masses due to the presence of stronger intermolecular
hydrogen bonding as shown below.

19. 4. Melting points of aliphatic monocarboxylic acids shows alternation or oscillation effect,
i.e., the m.p. of an acid with even number of carbon atoms is higher than the next lower
and next higher homologue containing odd number of carbon atoms. This is because, in
case of acids with even number of carbon atoms, the terminal -CH3 and -COOH groups lie
on the opposite sides of the zig-zag chain. As a result, they get closely packed in the
crystal lattice.
5. Glacial acetic acid is completely pure acetic acid and represents the solid state of
acetic acid. Below 16.6°C temperature pure acetic acid is converted into ice like solid
hence it is called glacial acetic acid.
(i) Acidity

20. The strength of the acid is expressed in terms of the dissociation
constant (Ka), also called acidity constant. A stronger acid has
higher Ka but lesser pKavalue (pKa == -log Ka).
The electron releasing substituents (+1 effect) decrease the acidic
strength of the carboxylic acids by destabilising the carboxylate
ion.